(1) Field of the Invention
The present invention relates generally to a method and apparatus for measuring and monitoring physiological events in humans and animals, and more particularly to a non-contact method and apparatus for continuously measuring and monitoring physiological events in humans or animals using a laser Doppler vibrometer to create waveforms which are directly related to the physiological events.
(2) Description of the Prior Art
For decades, there has been a long felt but unsolved need to continuously and accurately measure physiological events such as blood pressure without making contact with the patient. For many patients, such as burn victims, neonates, and for patients who need to be monitored without disturbing sleep or rest, the ability to accurately monitor blood pressure waveforms without contact has long been desired, but never accomplished.
Invasive monitoring systems, using intra-arterial catheters containing miniature pressure transducers are implemented for continuous monitoring of arterial pressure waveforms, as well as determining blood pressure values throughout the cardiac cycle. However, due to the requirement of inserting these sensors into the arterial system, the patient may be placed in distress.
An extremely well known non-invasive, contact method of measuring blood pressure uses a sphygmomanometer cuff wrapped around the subject's arm above the elbow. As the cuff is being inflated, a stethoscope is utilized to hear the sounds that correspond to the systolic and diastolic end-points. These end-points assist in determining the corresponding blood pressure values. This method provides only systolic and diastolic pressure values for a moment in time and does not provide time-continuous pressure measurements.
Methods for continuously monitoring blood pressure that do not require insertion of sensors into an artery, i.e., non-invasive methods, have been developed within the last decade. For instance, U.S. Pat. No. 5,363,855, which is discussed below, discloses a non-invasive means for continuously monitoring blood pressure. However, contact must be made with the subject and so a non-contact method for measuring blood pressure is not disclosed. Other prior art teachings as listed below, disclose various means for measuring blood flow velocity, blood oxygen saturation, and the like, by non-contact means. However, such techniques are complicated to set up and have not been able to provide sufficient accuracy or definition of the timing of the blood pressure waveform so as to be of any significant benefit in analysis of the cardiac cycle beyond very roughly indicating basic features such as the heart rate. For instance, such techniques have never been utilized to accurately detect the timing of the dicrotic notch within the arterial blood pressure waveform, and may be incapable of doing so.
Continuous recording of an accurate blood pressure waveform permits time series data analysis of the cardiac cycle. Analysis of the arterial pressure waveform identifies important events in the cardiac cycle, e.g., the timing of peak systole, the dicrotic notch, the pre-ejection period (PEP), the left ventricular ejection time (LVET), pulse rate, etc. Information about the systolic time intervals is useful in assessing cardiac condition and various disease states, including left ventricular failure, myocardial infarction, coronary artery disease, and valve disorders.
The time intervals of the various stages of the cardiac cycle are also observed for changes under cardiac disease conditions and pharmacological influence. For example, continuous monitoring of pre-ejection period and left ventricular ejection time ratios may be utilized to test the effects of drugs, exercise, or other stimuli, whereby an increase or decrease in the ratio may indicate an improvement or worsening of systolic efficiency.
The three basic systolic time intervals are the pre-ejection period (PEP), left ventricular ejection time (LVET) and total electromechanical systole (QS2). Linear relationships between heart rate (HR) and the duration of the systolic phases of the left ventricle (LV) have been derived by observation. These following equations have been utilized in the prior art to predict the durations of the systolic time intervals for normal patient observations based on the heart rate alone:PEP=−0.0004*HR+0.126  (1)LVET=−0.0016*HR+0.394  (2)QS2=−0.020*HR+0.522  (3)
The dicrotic notch as observed on a blood pressure waveform indicates the occurrence of the closure of the aortic valve and marks the end of left ventricular ejection. This event represents the end of the systolic phase and the start of diastole and left ventricular relaxation. The location of the dicrotic notch on a blood pressure waveform can be used for evaluating the above listed linear regression equations that may be utilized to predict the systolic time interval as a function of heart rate. The regression equations are expected to deviate for patients with cardiac dysfunction.
The following U.S. patents describe various prior art systems related to the above discussed problems but do not satisfy the long felt but unsolved need for non-contact blood pressure waveform monitoring.
U.S. Pat. No. 5,778,878, issued Jul. 14, 1998, to K. Kellam, discloses a laser Doppler technique to determine the velocity of blood cells in skin or other tissue capillaries. A laser beam is focused on to a capillary by means of a lens, mirror and beam splitter system. Measurement of the velocity of the blood cells in a direction substantially perpendicular to the surface of the tissue is effected by detecting directly back-scattered radiation.
U.S. Pat. No. 5,363,855, issued Nov. 15, 1994, to Drzewiecki et al., discloses a pressure waveform monitor that noninvasively monitors the pressure waveform in an underlying vessel such as an artery. The apparatus comprises at least one continuous, relatively thin and narrow diaphragm mounted in a housing to be placed on the tissue overlying the vessel of interest. The diaphragm is longer than the diameter of the vessel for purposely monitoring pressure in the tissue adjacent to the vessel of interest. The device also comprises deformation sensor means for measuring deformation of the diaphragm both over the vessel and adjacent to the vessel, and signal processing means for combining the waveform of the vessel as monitored by the part of the diaphragm over the vessel with the waveforms of adjacent tissue to accurately determine the actual pressure waveform in the vessel.
U.S. Pat. No. 5,361,769, issued Nov. 8, 1994, to G. Nilsson discloses a method and a system for reducing the distance-dependent amplification factor when measuring fluid flow movements with the aid of an image-producing laser-Doppler technique, in particular when measuring blood perfusion through tissue. A laser beam source directs a laser beam onto a measurement object, which scatters and reflects the beam. The reflected light is received by a detector that senses the broadening in frequency caused by the Doppler effect. One or more lenses are placed in the path of the beam and are intended to maintain constant the number of coherence areas on the detecting surface of the detector and independent of the distance between detector and measurement object.
U.S. Pat. No. 5,280,789, issued Jan. 25, 1994, to R. A. Potts, discloses an apparatus for vertically aligning a given point on a pressure transducer unit with a desired point on a patient comprising a light source, a housing adapted to contain the light source, and at least one leveling tube having a leveling axis that is substantially parallel to the light beam. The leveling tube comprises a closed transparent envelope containing a liquid and a bubble of gas, and lines formed on the envelope, where the leveling axis is substantially horizontally aligned when the bubble of gas is located between the two lines. The apparatus includes an indicating mark formed on the housing means where the beam of light is vertically aligned with the given point on the transducer. A locking system selectively locks the housing means to prevent movement thereof relative to the transducer unit when the beam of light is both horizontally aligned and vertically aligned with the given point on the transducer unit. To vertically align the given point with the desired point, one of the transducer units and the patient are vertically displaced relative to the other until the light source causes light to reflect off of the patient at the desired point.
U.S. Pat. No. 4,166,695, issued Sep. 4, 1979, to Hill et al., discloses a means for measuring blood flow in retinal blood vessels by directing laser radiation along an optical path into the eye and onto a blood vessel. Laser radiation reflected off moving blood corpuscles is directed back along the optical path and into a detector. This reflected laser radiation is mixed with a proportion of the original laser signal to determine the Doppler shift produced by the moving blood corpuscles and hence blood velocity.
U.S. Pat. No. 5,995,856, issued Nov. 30, 1999, to Mannheimer et al., discloses monitoring of physiological parameters of a patient through the use of optical systems that do not require direct physical contact with the patient. The method and apparatus relate primarily to pulse oximetry for monitoring pulse rate and arterial blood oxygen saturation. However, the apparatus and method of this invention are applicable to any form of optical detection of the physiological parameters in which light of any wavelength, visible or invisible, is directed from a remote instrument into a patient at a first imaging site, and subsequently collected at a second site spaced from the first site.
U.S. Pat. No. 6,007,494, issued Dec. 28, 1999, to Zenner et al., discloses a device for determining data on auditory capacity wherein the device preferably has non-contact means for measuring vibrations of the middle-ear ossicles and/or the tympanic membrane by means of electromagnetic waves. The electromagnetic waves used for the measurement are input by means of a microscope, in particular an optical microscope. This microscope can be modular in design, and a module can be provided for the input of a laser beam. The invention also concerns a method of determining data on auditory capacity wherein the method calls for the vibration of the middle ear and/or the eardrum to be measured by means of electromagnetic waves and, from the measurement signals thus obtained, the contributions to the total signal by the middle ear and/or the eardrum determined in at least one processing step.
The Journal of Biomedical Engineering, 4(2): 142–8, 1982, by Brown et al. teaches that a rather complex light emitting diode sensor (LED) has sufficient resolution to detect an arterial pulse.
The above-discussed systems do not disclose a convenient and completely non-contact means for accurately and continuously monitoring blood pressure or creating blood pressure waveforms. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.